A to Z listing
Search this site
Corrosion Glossary - A
- Abatement: either removal of the painted
surface, covering the painted surface with an impermeable surface, or covering
surface with heavy-duty coating or encapsulant.
- Abradable coatings:
coatings which are designed
to be abraded by a mating surface to form a tight gas or air seal, while retaining
good erosion resistance
- Abrasive: material such as sand, crushed
chilled cast iron, crushed steel grit, aluminum oxide, silicon carbide, flint,
garnet, of crushed slag used for cleaning or surface roughening.
- Abrasive blasting: see preferred term
- Abrasive wear: wear due to hard particles
or hard protuberances forced against and moving along a solid surface.
- Absolute pressure: pressure above zero
pressure; the sum of the gauge and atmospheric pressures.
- Absorb: to take in and engulf wholly.
- Absorbent: A material, usually a porous
solid, which takes another material into its interior. When rain soaks into
soil, the soil is an absorbent.
- Absorption: a process in which molecules
are taken up by a liquid or solid and distributed throughout the body of that
liquid or solid. Compare with adsorption.
- Absorption: The process in which one
substance is taken into the body of an absorbent.
- Acetylene: unsaturated aliphatic hydrocarbon
gas. Used as a fuel gas in combustion thermal spray processes, welding and cutting.
Acetylene has the highest flame temperature and requires the smallest amount
of oxygen to form a neutral flame.
- AC impedance: See
electrochemical impedance spectroscopy
- Acicular ferrite: a highly substructured
non-equiaxed ferrite formed upon continuous cooling by a mixed diffusion and
shear mode of transformation that begins at a temperature slightly higher than
the transformation temperature range for upper bainite. It is distinguished
from bainite in that it has a limited amount of carbon available thus, there
is only a small amount of carbide present.
- Acid: a substance which releases hydrogen
ions when dissolved in water. Most acids will dissolve the common metals and
will react with a base to form a neutral salt and water. (see
- Acid cleaning: the process of cleaning
the interior surfaces of steam generating units by filling the unit with dilute
acid accompanied by an inhibitor to prevent corrosion, and subsequently draining,
washing and neutralizing the acid by a further wash of alkaline water.
- Acid embrittlement: a form of
that may be induced in some metals by acid.
- Acid mine drainage: drainage of water
from areas that have been mined for coal or other mineral ores; the water has
low pH, sometimes less than 2.0,
because of its contact with sulfur-bearing material.
- Acid rain:
atmospheric precipitation with a pH
below 3.6 to 5.7. Burning of fossil fuels for heat and power is the major factor
in the generation of oxides of nitrogen and sulfur, which are converted into
nitric and sulfuric acids washed down in the rain. See also atmospheric corrosion.
- Acidity: The quantitative capacity of
water or a water solution to neutralize an alkali or base. It is usually measured
by titration with a standard solution of sodium hydroxide and expressed in terms
of its calcium carbonate equivalent. (See mineral acidity, total acidity, carbon
Acoustic emission: acoustic
emissions are sound or ultrasound pulses generated during crack initiation or
propagation in materials and coatings as a result of being subjected to stress.
Acoustic emissions can be detected by transducers.
rubber: resin binder
produced from the copolymerisation of styrene and acrylic monomers.
a synthetic resin used in high-performance
water-based coatings and produced by polymerizing various acrylic or acrylate
monomers. This resin is used in single pack coatings which have good color and
- Activated carbon: a water treatment
medium, found in block, granulated, or powdered form, which is produced by heating
carbonaceous substances (bituminous coal or cellulose-based substances such
as wood or coconut shell) to 700oC or less in the absence of air
to form a carbonized char, and then activating (oxidizing) at 800 to 1000oC
with oxidizing gases such as steam and carbon dioxide (oxygen is never used
as the oxidizing gas because its reaction with the carbon surface is too rapid
and violent) to form pores, thus creating a highly porous adsorbent material.
- Activated silica: A material usually
formed from the reaction of a dilute silicate solution with a dilute acid. It
is used as a coagulant aid.
- Activation: the changing of a passive
surface of a metal to a chemically active state. Contrast with passivation.
the overpotential associated with the charge-transfer reaction elementary step
in the overall electrode reaction.
- Active: the negative direction of electrode
potential (Also used to describe corrosion and its associated potential range
when an electrode potential is more negative than an adjacent depressed corrosion
rate [passive] range.
- Activity: a measure of the chemical
potential of a substance, where chemical potential is not equal to concentration,
that allows mathematical relations equivalent to those for ideal systems to
be used to correlate changes in an experimentally measured quantity with changes
in chemical potential.
- Activity (ion): the ion concentration
corrected for deviations from ideal behavior. Concentration multiplied by activity
- Activity coefficient: a characteristic
of a quantity expressing the deviation of a solution from ideal thermodynamic
behavior; often used in connection with electrolytes.
- Actuarial analysis: statistical analysis
of failure data to determine the age-reliability characteristics of an item.
- Asset management: the systematic planning
and control of a physical resource throughout its life. This may include the
specification, design, and construction of the asset, its operation, maintenance
and modification while in use, and its disposal when no longer required.
- Adhesion: a binding force that holds
together molecules of substances whose surfaces are in contact or near proximity.
(Paint) The ability of dry paint to attach to and remain fixed on the
surface without blistering, flaking, cracking or being removed by tape.
- Adhesive strength: the magnitude of
attractive forces, generally physical in character, between a coating and substrate.
Two principle interactions that contribute to the adhesion are van der Waals
forces and permanent dipole bonds.
- Adhesive Wear: wear due to localized
bonding between contacting solid surfaces leading to material transfer between
the two surfaces or the loss from either surface.
- Adsorb: to take in on the surface.
- Adsorbent: A material, usually solid,
capable of holding gases, liquids and/or suspended matter at its surface and
in exposed pores. Activated carbon is a common adsorbent used in water treatment.
- Adsorption: the physical process occurring
when liquids, gases, or suspended matter adhere to the surfaces of, or in the
pores of, an adsorbent medium. Adsorption is a physical process which occurs
without chemical reaction.
- Aeration: the process whereby water
is brought into intimate contact with air by spraying or cascading, or air is
brought into intimate contact with water by an air aspirator or by bubbling
compressed air through the body of water.
- Aeration cell: an oxygen concentration
cell; an electrolytic cell resulting from differences in dissolved oxygen at
two points. Also see differential aeration cell.
- Aerobic: an action or process conducted
in the presence of air, such as aerobic digestion of organic matter by bacteria.
- Aerosol: aerosols are tiny particles
suspended in the air. Some occur naturally, originating from volcanoes, dust
storms, forest and grassland fires, living vegetation, and sea spray. Human
activities, such as the burning of fossil fuels and the alteration of natural
surface cover, also generate aerosols. (Paint) A product that uses compressed
gas to spray the coating from its container.
- Age hardening: hardening by aging, usually
after rapid cooling or cold working.
- Agglomerate: several particles adhering
- Agglomerated powder: a powder made up
- Air Cure: One method by which liquid
coatings cure to a dry film. Oxygen from the air enters the film and cross-links
the resin molecules. Also called "Air Dry" and "Oxidizing."
- Air spray: a system of applying paint
in the form of tiny droplets in the air. The paint is broken down into droplets
(atomized) by a spray gun as a result of being forced into a high velocity air
- Air-assisted airless: an airless system
that uses small amounts of air to help atomize the paint in conjunction with
airless spray. Used to get rid of the nagging problem of “Rabbit Ears”.
- Airless spray: a system of applying
paint in which the paint, under high pressure, is passed through a nozzle and
is broken into droplets (atomized) when the paint enters the lower pressure
composite wrought product comprised of an aluminum alloy core having on one
or both surfaces a metallurgically bonded aluminum or aluminum alloy coating
that is anodic to the core and thus electrochemically protects the core against
- Algae: Small primitive plants containing
chlorophyll, commonly found in surface water. Excessive growths may create taste
and odor problems, and consume dissolved oxygen during decay.
- Aliphatic: organic compounds (hydrocarbon)
in which carbon atoms are arranged in an open or straight chain more commonly
- Alkali: A group of water soluble mineral
compounds, usually considered to have moderate strengths as bases (as opposed
to the caustic or strongly basic hydroxides, although this differentiation is
not always made). In general, the term is applied to bicarbonate and carbonate
compounds when they are present in the water or solution. (See alkali, base.)
- Alkaline: a) Having properties of an
alkali and, b) Having a pH greater
- Alkaline cleaner: a material blended
from alkali hydroxides and such alkaline salts as borates, carbonates, phosphates,
or silicates. The cleaning action may be enhanced by the addition of surface-active
agents and special solvents.
- Alkalinity: The quantitative capacity
of a water or water solution to neutralize an acid. It is usually measured by
titration with a standard acid solution of sulfuric acid and is expressed in
terms of its calcium carbonate equivalent. (See alkali, base,
- Alkyd: synthetic polyester resin modified
with oil. Coating that contains alkyd resins in the
binder. The original alkyd resins
were made by co-polymerizing phtalic anhydride with glycerol, to give a brittle
cross. Linked polymer.
- Alkyl benzene sulfonate: A term applied
to a family of branched chain chemical compounds, formerly used as detergents,.
Sometimes called "hard" detergents, because of their resistance to biological
degradation, these compounds have been largely replaced with linear alkyl sulfonate
(LAS) which are more readily degraded to simpler substances.
- Alloy steel:
steel with modified properties made
by combining iron with one or more elements in addition to carbon. Alloys change
the properties of the steel making it, for example, harder, more formable, or
more corrosion resistant, depending on the combination and amounts of alloys
- Alodine (or Irridite): a dichromate
conversion coating used with aluminum. Clear, yellow, or gold in appearance,
alodine's main purpose is to prepare aluminum for painting. (see hexavalent
- Alpha iron: the body-centered cubic
form of pure iron, stable below 910ºC.
test: a corrosion
test in which the specimens are intermittently exposed to a liquid medium
at definite time intervals.
- Alum: A common name for aluminum sulfate,
used as a coagulant.
- Aluminizing: forming of an aluminum
or aluminum alloy coating on a metal by hot dipping, hot spraying, or diffusion.
- Aluminizing (gas): high temperature
pack or gaseous diffusion of aluminum
into the surface of a component to enhance high temperature corrosion and oxidation
- Aluminizing (hot dip): an aluminum coating
process based on submersion in liquid metal, usually with a strip steel product
being continuously fed through the bath. Provide galvanic corrosion protection.
- Aluminizing (thermal spray): thermal
sprayed coatings of aluminum usually on substrates of steel or nickel chromium
alloys which are subsequently heat treated to aluminize the surface.
- Aluminum ion plating: the deposition
of aluminum by a vacuum evaporative process that provides galvanic corrosion
resistance. Normally given a passivation treatment.
- Amide: a functional group which can
act as an epoxy resin curing agent.
- Amoeba: A small, single-celled animal
- Amorphous: non-crystalline, or devoid
of regular structure.
- Amphoteric: a term applied to oxides
and hydroxides which can act basic toward strong acids and acidic toward strong
alkalis. Substances which can dissociate electrolytically to produce hydrogen
or hydroxyl ions according to conditions.
- Anaerobic: in the absence of air or
unreacted or free oxygen.
- Angstrom unit: A unit of length equal
to one ten-billionth of a meter.
- Anion: a negatively charged ion.
- Anion exchange: An ion exchange process
in which anions in solution are exchanged for other anions from an ion exchanger.
In demineralization, for example, bicarbonate, chloride and sulfate anions are
removed from solution in exchange for a chemically equivalent number of hydroxide
anions from the anion exchange resin.
- Annealing: a process involving heating
and cooling designed to effect: (1) softening of a cold-worked structure by
re-crystallization of grain growth or both; (2) softening of an age-hardened
alloy by causing a nearly complete precipitation of the second phase in a relatively
coarse form; (3) softening of certain age-hardenable alloys by dissolving the
second phase and cooling rapidly enough to obtain a supersaturated solution;
(4) relief of residual stress.
- Anode: the electrode of an
electrolytic cell at which
oxidation is the principal reaction. (Electrons flow away from the anode in
the external circuit. It is usually the electrode where corrosion occurs and
metal ions enter solution.
- Anode corrosion efficiency: the ratio
of the actual corrosion (weight loss) of an anode to the theoretical corrosion
(weight loss) calculated by Faraday's
law from the quantity of electricity that has passed.
- Anodic cleaning: electrolytic cleaning
in which the work is the anode. Also called reverse-current cleaning.
- Anodic coating: a coating that becomes
the anode in an electrochemical cell with the substrate (cathode). The only
metals in common use for thermal spraying which are anodic to iron and steel
are zinc and
- Anodic inhibitor:
an inhibitor that reduces the corrosion rate by acting on the anodic (oxidation)
- Anodic polarization: the change of the
electrode potential in the noble (positive) direction due to current flow (See
- Anodic protection: a technique to reduce
corrosion of a metal surface under some conditions by passing sufficient to
it to cause its electrode potential to enter and remain in the passive region;
imposing an external electrical potential to protect a metal from corrosive
attack. (Applicable only to metals that show active-passive behavior.) Contrast
with cathodic protection.
the production of an oxide layer on aluminum alloys. The process is electrolytic,
a typical electrolyte being sulfuric acid. Treatment at room temperature produces
thin, decorative layers with some corrosion protection. Treatment at lower temperatures
can produce harder, thicker layers (up to 100µ) with wear resistance. They can
be post sealed to give improved corrosion resistance.
- Anolyte: the electrolyte adjacent to
the anode of an electrolytic
- Anti-fouling paint: a
paint formulated to prevent fouling
of under-water structures, such as the bottoms of ships; refers to the prevention
of marine organism's attachment or growth on a submerged metal surface, generally
through chemical toxicity caused by the composition of the metal or coating
an addition agent for electroplating
solutions to prevent the formation of pits or large pores in the electrodeposits.
- Apparent density: the weight of a unit
volume of powder or coating.
- Apparent hardness: the value obtained
by testing a coating or sintered material with standard indentation hardness
equipment. Since the reading reflects a composite of pores and solid material,
(which may be particles relatively poorly bonded together) it is usually lower
than that of an equivalent solid wrought or cast material. Not to be confused
with particle hardness. (See also microhardness)
- Aqueous: pertaining to water; an aqueous
solution is made by using water as a solvent.
- Aquifer: A layer or zone below the surface
of the earth which is capable of yielding a significant volume of water.
- Arc wire spraying: a thermal spray process
where two electrically conducting wires are brought together to form an electric
arc. Molten material formed in the arc is projected by a compressed gas stream
towards the work piece to form a coating.
- Argon (Ar): monatomic noble gas, atomic
number 18, one of the most inert elements. Commonly used as a plasma gas for
plasma spraying and providing inert environments for many processes.
- Aromatic: a type of solvent based on
Benzene ring molecules (e.g. Benzene, Xylene,
- Arrester: a device to impede the flow
of large dust particles or sparks from a stack, usually screening at the top.
- Artificial aging: aging above room temperature.
See also aging. Compare with natural aging.
- Ash: the incombustible inorganic matter
in the fuel.
- ASME: the American Society of Mechanical
Society for testing of Materials
- Atmospheric air: air under the prevailing
- Atmospheric corrosion: the gradual degradation
or alteration of a material by contact with substances present in the atmosphere,
such as oxygen, carbon dioxide, water vapor, and sulfur and chlorine compounds.
- Atmospheric pressure: the barometric
reading of pressure exerted by the atmosphere. At sea level 0.101 Mpa or 760
mm of mercury.
- Atom: the smallest particle of an element
that can exist either alone or in combination.
- Atomic-hydrogen welding: arc welding
in which molecular hydrogen, passing through an arc between two tungsten or
other suitable electrodes, is changed to its atomic form and then re-combines
to supply the heat for welding.
- Atomization: 1) The dispersion of a
molten material into particles by a rapidly moving gas or liquid stream or by
mechanical dispersion. 2) The formation of tiny droplets of liquid as in the
paint spraying process. Atomization is usually caused by turbulence in an air
stream, or a sudden drop in pressure.
- Atomized powder: a powder produced by
the dispersion of a molten material into particles by a rapidly moving gas or
liquid stream or by mechanical dispersion.
- Attrition: In water treatment, the process
in which solids are worn down or ground down by friction, often between particles
of the same material. Filter media and ion exchange materials are subject to
attrition during backwashing, regeneration and service.
- Austenite: a solid solution of one or
more elements in face-centered cubic iron. Unless otherwise designated (such
as nickel austenite), the solute is generally assumed to be carbon.
- Austenitic: the name given to the face-centered
cubic crystal structure (FCC) of ferrous metals. Ordinary iron and steel has
this structure at elevated temperatures; also certain
(300 series) have this structure at room temperature.
- Austenitizing: forming austenite by
heating a ferrous alloy into the transformation range (partial austenitizing)
or above the transformation range (complete austenitizing). When used without
qualification, the term implies complete austenitizing.
- Autoclaving: the production of a stable,
protective oxide on steel parts by treatment in a pressurized, high temperature
steam containing atmosphere.
- Auxiliary electrode: the electrode in
an electrochemical cell
that is used to transfer current to or from a test electrode, usually made of
- Availability: the proportion of total
time that an item of equipment is capable of performing its specified functions,
normally expressed as a percentage. It can be calculated by dividing the equipment
available hours by the total number of hours in any given period.
- Average life: how long, on average,
a component will last before it suffers a failure. Commonly measured by Mean
Time Between Failures (MTBF).